The present invention relates to an improvement in the melt process for making prills from a melted substance.
Various prilling processes have been proposed for making prills from highly hydroscopic substances such as urea or from substances which are difficult to handle in powder form, thereby improving the flowability, preventing dust generation and decreasing their specific surface. Among these, the principal approach is the melt prilling process, and particularly employed widely is the so-called spray-prilling process in which a melted substance is showered or sprayed from the top of a tower by a nozzle with small holes or orifaces or by centrifugal force with a rotating disc, wherefrom prills are obtained either by drying or solidifying the dropping liquid particles through countercurrent gas introduced from the bottom of tower, or by dropping said liquid particles into a medium inert to said melted substance.
In such a prilling process, smooth passage of the melted substance through the nozzle is indispensable. In this connection pre-heating of the nozzle is indispensable, especially important so as to prevent viscosity increase of the melted substance in the vicinity of the nozzle at the start of the prilling process. In the absence of pre-heating, or in case of insufficient pre-heating, the melted substance present in the vicinity of nozzle is cooled, resulting in a viscosity increase or eventually solidification so as to clog the nozzle and thereby render the operation impossible. In such case, the substance, once it solidifies in the nozzle, does not easily melt not even by forced heating and the nozzle has to be removed for cleaning or replaced by a new nozzle followed by resumed pre-heating before the prilling operation can be started. Also, even if the nozzle is not clogged completely, partial clogging of nozzle will result in the formation of relatively large dripping liquid particles which drop in an unsolidified state and stick to the bottom of the tower. The presence of such adhering unsolidified substance causes reheating by the heat accumulated therein, of other particles completely solidified, thereby resoftening such particles and thus forming a deposit on the bottom of tower, rendering the discharge of prills difficult. It therefore frequently becomes necessary to interrupt the operation and remove the deposit formed on the bottom of tower. Such deposit, being a defective product, not only reduces the yield of production but also hinders continuous operation. Furthermore there may be obtained flat or fiber-like products depending upon the clogged state of the nozzle, since solidification proceeds before the surface tension of liquid particles becomes sufficiently effective.
In general the nozzle is connected to a spray chember which is heated to maintain the temperature thereof by steam or other suitable heat transfer liquid, but the nozzle is hardly heated by said heating alone. It is to be noted that such heating means is easily available but is difficult to mount on the nozzle itself. However a complete pre-heating of the nozzle is indispensable as explained in the foregoing since the trouble at the start of the prilling process has to be avoided.
Therefore, various methods for pre-heating the nozzle have been proposed, such as:
(1) external pre-heating by infrared irradiation; PA1 (2) external pre-heating by blowing of hot air; and PA1 (3) pre-heating by resistor type heater etc.
However these pre-heating methods, all depending on external heating and mostly utilizing heat of radiation and thus having low heating efficiency which is further reduced by air flow from the bottom of the tower, were unable to exhibit a sufficient heating effect even after a prolonged pre-heating time. Also installation of a heating device within the tower inevitably results in a complicated structure for the prilling apparatus which often affects the prilling operation and renders maintenance cumbersome. Also a heater often results in partial heating of the nozzle which can become a cause of trouble in the prilling operation, and, besides, the use of an inflammable heat source in the tower is not desirable from the standpoint of safety.
The object of the present invention, therefore, is to provide an improved prilling process of high efficiency allowing easy and assured start of prilling, and an economical operation using a simple apparatus.